In a Coordinate Multi-Point (CoMP) system, two or more base stations (BSs) or cells cooperatively communicate with a user equipment (UE) in order to improve communication performance between a BS (cell or sector) and a UE located in a shadow area. A conventional CoMP scheme may be divided into a Multiple-Input Multiple-Output (MIMO) CoMP-Joint Processing (JP) scheme using data sharing and a CoMP-Coordinated Scheduling/Beamforming (CoMP-CS/CB) scheme.
In the CoMP-JP scheme, a UE may instantaneously and simultaneously receive data from BSs which belong to the same cluster and perform CoMP and combine signals received from the BSs so as to improve reception performance.
In contrast, in the CoMP-CS/CB scheme, a UE may instantaneously receive data from one BS. Scheduling or beamforming is performed such that interference with other BSs within a cluster to which the UE belongs is minimized.
In order to perform CoMP, a CoMP cluster which is a set of BSs which can perform a CoMP operation needs to be preferentially set. A conventional method of constituting a cluster includes a method of constituting and managing a cluster by a BS system or a BS control system and a method of constituting and managing a cluster by a UE.
FIG. 1 is a diagram showing the case where a BS control system constitutes a cluster.
Referring to FIG. 1, first, each UE measures interference between neighboring BSs and the UE, averages the interference during a predetermined period, and transmits the average to the BS control system. The BS control system determines interference between the BSs and the UE using the transmitted information and collects BSs which are expected to have high performance improvement if CoMP is used, thereby constituting a cluster. The number of BSs within one cluster may be changed and different clusters differ in the number of BSs included therein. In FIG. 1, one cluster includes three BSs.
FIG. 2 is a diagram showing the case where a UE constitutes a cluster.
Referring to FIG. 2, a UE 1 and a UE 2 measure interference between neighboring BSs and the respective UEs and average the interference during a predetermined period of time. Each UE determines interference between the BSs and the UE based on the average, and collects BSs which are expected to have high performance improvement if CoMP is used, thereby directly constituting a cluster. Then, each UE exchanges the constituted cluster information with a BS control system. In FIG. 2, it is assumed that one cluster includes three BSs, similarly to FIG. 1. The number of BSs belonging to one cluster may be changed. Different clusters may differ in the number of BSs included therein.
If only a BS control system constitutes a cluster, the BS control system intermittently constitutes a new cluster by applying a variation in data transfer amount of each cluster according to movement of a UE. However, if only the BS control system constitutes and manages all clusters within in the network, there is a limit to the degree of a performance improvement obtained using CoMP.
If only each of UEs (UE 1 and UE 2) constitutes a cluster, each of the UE 1 and UE 2 may transmit information regarding a cluster variation according to movement of each UE to a BS using resources of a specific region. At this time, since respective resource allocation regions of the UEs may overlap with each other, cooperation between the UEs is necessary. If each of the UEs constitutes a cluster, scheduling overhead severely increases.
As a UE moves, handoff may occur. Handoff means that, when a UE moves from a BS (cell or sector) to another BS (cell or sector), a channel between the UE and the existing BS is changed to a channel between the UE and the new BS, in order to maintain communication. Handoff may be broadly divided into a hard handoff scheme for breaking the existing channel and then performing connection to a new BS and a soft handoff scheme for maintaining a channel between two BSs (cells or sectors) using a Code Division Multiple Access (CDMA) scheme.
In general, a UE may classify and manage cell IDs within a system, for handoff, which will now be briefly described. An active set includes cells to which a forward data channel (or a downlink data channel) is allocated, that is, cells which are currently performing communication. A candidate set includes cells which are not included in the active set but have sufficient field strengths. A neighbor set includes peripheral cells which may sufficiently belong to a candidate set and are notified through a neighbor list message. A remaining set includes other cells.
A UE uses thresholds T_ADD, T_DROP, T_COMP and T_TDROP, for classification. T_ADD is a reference value for changing a cell from the neighbor set to the candidate set. T_DROP is a reference point for operating T_TDROP in the active set and T_COMP is a reference value for comparing field strength of a signal received from the active set. T_TDROP is a timer which operates when the field strength of a signal received from a cell becomes equal to or less than the T_DROP value. Hereinafter, a handoff process of a conventional UE will be described.
FIG. 3 is a diagram showing handoff state among a BS 1, a BS 2 and a BS 3.
Referring to FIG. 3, it is assumed that a UE 1 which receives a data channel allocated by the BS 1 and performs data communication with the BS 1 enters a handoff area between the BS 1 and the BS 2. That is, it is assumed that the BS 1 belongs to the active set of the UE 1 and the remaining cells belong to the neighbor set of the UE 1.
If the field strength of a signal received from the BS 2 is greater than the T_ADD value, the UE 1 transmits information about the field strength of a signal received from each BS through a pilot strength measurement message (PSMM) to the BS 1, in order to transfer the BS 2 from the neighbor set to the candidate set. The BS 1 transmits a handoff direction message (HDM) to the UE 1 so as to include the BS2 in the candidate set.
Thereafter, if the field strength of the signal received from the BS 2 is greater than the field strength of the signal received from the cell in the active set by the T_COMP value or more, the UE 1 transmits a PSMM to the BS 1 in order to transfer the BS 2 from the candidate set to the active set, and the BS 1 and the BS 2 transmit an HDM to the UE 1 so as to include the BS 2 in the active set. Then, the active set includes the BS 1 and the BS 2 and the UE 1 may receive data through a combination of data channels of the BS 1 and the BS 2.
Thereafter, if the UE 1 gradually moves toward the BS 2 and thus the field strength of the signal received from the BS 1 becomes less than the T_DROP value, the T_TDROP timer begins to operate, and, if the T_TDROP timer has expired and the field strength of the signal received from the BS 1 is less than the T_DROP value, the BS 1 is transferred from the active set to the neighbor set and handoff from the BS 1 to the BS 2 is completed.
It is assumed that the UE 1 simultaneously receives data from the BS 1 and the BS 2 and moves toward the BS 3. If the field strength of the signal received from the BS 3 is gradually increased and becomes greater than the T_ADD value, the UE 1 transmits a PSMM to the BS 1 and the BS 2 in order to transfer the BS 3 from the neighbor set to the candidate set. The BS 1 and the BS 2 transmit an HDM to the UE 1 so as to include the BS 3 in the candidate set.
Thereafter, if the field strength of the signal received from the BS 3 is greater than that of the active cell by the T_COMP value, the UE 1 transmits a PSMM to the BS and the BS 2 in order to transfer the BS 3 from the candidate set to the active set, and the BS 1 and the BS 2 transmit an HDM to the UE 1 so as to include the BS 3 in the active set. Then, the active set including the BS 1, the BS 2 and the BS 3 and the UE 1 may receive data through a combination of data channels of the BS 1 and the BS 2.
Thereafter, if the UE 1 gradually moves toward the BS 3, only the BS 3 is included in the active set. However, when a resource allocation state of the BS 3 is considered, it may difficult to perform handoff to the BS 3. Since handoff is conventionally determined based only on the pilot strength measurement of each BS, a problem may occur. In the related art, only handoff to one BS is considered.
In addition, a method of selecting an adequate CoMP scheme from among various CoMP schemes according to the current state of a UE is not yet defined.